Can I collect my ex-husbands SSI benefit even though his parental rights were terminated?

It is unlikely you can get anywhere with this.  First off there is a three-year limitations on asking for support.  So unless there was already a support order in place on the date of the adoption, the bio father was only on the hook for three years back support at that time, and every day that passed without you filing for an order for back support reduced the amount of back support he could be ordered to pay.  So as of the three-year anniversary of the adoption any claim for back support was gone.  If it is less than three years since the adoption, you can file now for a support claim against his estate, if he left anything, but that's it.

If you did already have an existing support order at the time of adoption, again your claim is against the estate at this point.  If the bio father had been receiving SSI payments while he was alive, you could have garnished those payments just like wages, but SSI doesn't pay anything to the decedent when he dies, nor to his estate, only survivor benefits.  So there is nothing from SSI owed to the bio father for you to attach.  And of course your son's rights to survivor benefits transferred from the bio father to your husband when the adoption happened, so your son isn't entitled to survivor benefits unless his legal father dies while he qualifies.